Pulp and paper sludge (a byproduct of primary pulping operations, recycle streams or waste paper pulping and the like), as well as the products of its incineration, represent an environmental and disposal problem for manufacturers of pulp and paper. Generally, pulp and paper sludge is unsuitable for paper making, although it generally contains the same components--cellulose, lignin, hemicellulose, calcium carbonate, clay, and other inorganic components--as those present in the paper pulp itself.
Paper sludge has traditionally been disposed of by landfilling, composting, incorporation into cement, and incineration. The latter option, in turn, creates another problem, namely, disposal of the resulting ash, which often makes up to 50% (and sometimes as much as 80% or higher) of the volume of the sludge itself. Calcium carbonate, in the form of precipitated calcium carbonate (PCC) or ground calcium carbonate (GCC), typically constitutes 20% and up to 75% of dry sludge content. Calcium carbonate is a natural carbonate which is loaded, typically together with clay, into paper as a coating and filler to improve the mechanical characteristics as well as the appearance of paper. Despite their natural abundance, calcium salts are generally expensive products because of the difficulties and expenses of their purification from natural mineral deposits. For instance, paper-quality PCC is typically produced from natural limestone via extensive processing including the calcination of limestone in an industrial kiln (to produce either a calcitic or a dolomitic lime), slaking, slurrying, carbonating, and a number of refining steps.
Unfortunately, the inorganic content of sludge and sludge-derived ash is generally largely or totally wasted. At best, the prior art describes utilization of incineration ash for production of low-end, impure products of limited market value. For example, Sohara ("Recycling Mineral Fillers from Deinking Sludges," Paper Recycling '96 Conf. 1996) details processing of such ash to precipitate calcium carbonate on the surface of the ash itself, which acts merely as nucleation center. In particular, an aqueous slurry of incineration ash is carbonated with carbon dioxide gas; calcium carbonate nucleates on the ash and grows during the precipitation reaction. The resulting mixture of precipitated calcium carbonate and ash still contains from 10% to 30% incineration ash, and represents an undifferentiated agglomeration of minerals and clay.
Nor does the prior art provide an effective way of isolating calcium salts from a mixture that includes salts of other cations, as, for example, from solubilized minerals from papermaking sludge. While various techniques for precipitating CaCO.sub.3 from aqueous suspensions of calcium hydroxide are known (see, e.g., U.S. Pat. Nos. 4,018,877, 4,124,688, 4,133,894, 4,157,379, 4,367,207, 4,714,603, 5,075,093, 5,120,521, 5,164,172, 5,232,678, 5,342,600, and 5,558,850), these are of limited usefulness in separating calcium from other salts.
Accordingly, lacking in the prior art is a cost-effective method of producing pure, high-grade calcium carbonate from papermaking sludge or ash derived therefrom.